The sleep apnea syndrome afflicts an estimated 1% to 5% of the general population and is due to episodic upper airway obstruction during sleep. Those afflicted with sleep apnea experience sleep fragmentation and intermittent, complete or nearly complete cessation of ventilation during sleep with potentially severe degrees of oxyhemoglobin desaturation. These features may be translated clinically into extreme daytime sleepiness, cardiac arrhythmias, pulmonary-artery hypertension, congestive heart failure and/or cognitive dysfunction. Other sequelae of sleep apnea include right ventricular dysfunction with cor pulmonale, carbon dioxide retention during wakefulness as well as during sleep, and continuous reduced arterial oxygen tension. Hypersomnolent sleep apnea patients may be at risk for excessive mortality from these factors as well as by an elevated risk for accidents while driving and/or operating potentially dangerous equipment.
Although details of the pathogenesis of upper airway obstruction in sleep apnea patients have not been fully defined, it is generally accepted that the mechanism includes either anatomic or functional abnormalities of the upper airway which result in increased air flow resistance. Such abnormalities may include narrowing of the upper airway due to suction forces evolved during inspiration, the effect of gravity pulling the tongue back to appose the pharyngeal wall, and/or insufficient muscle tone in the upper airway dilator muscles. It has also been hypothesized that a mechanism responsible for the known association between obesity and sleep apnea is excessive soft tissue in the anterior and lateral neck which applies sufficient pressure on internal structures to narrow the airway.
The treatment of sleep apnea has included such surgical interventions as uvulopalatopharyngoplasty, gastric surgery for obesity, and maxillo-facial reconstruction. Another mode of surgical intervention used in the treatment of sleep apnea is tracheostomy. These treatments constitute major undertakings with considerable risk of postoperative morbidity if not mortality. Pharmacologic therapy has in general been disappointing, especially in patients with more than mild sleep apnea. In addition, side effects from the pharmacologic agents that have been used are frequent. Thus, medical practitioners continue to seek non-invasive modes of treatment for sleep apnea with high success rates and high patient compliance including, for example in cases relating to obesity, weight loss through a regimen of exercise and regulated diet.
Recent work in the treatment of sleep apnea has included the use of continuous positive airway pressure (CPAP) to maintain the airway of the patient in a continuously open state during sleep. For example, U.S. Pat. No. 4,655,213 discloses sleep apnea treatments based on continuous positive airway pressure applied within the airway of the patient.
An early mono-level CPAP apparatus is disclosed in U.S. Pat. No. 5,117,819 wherein the pressure is measured at the outlet of the blower so as to detect pressure changes caused by the patient's breathing. The arrangement is such that the control motor is regulated by the microprocessor to maintain the pressure at constant level regardless of whether the patient is inhaling or exhaling.
Also of interest is U.S. Pat. No. 4,773,411 which discloses a method and apparatus for ventilatory treatment characterized as airway pressure release ventilation and which provides a substantially constant elevated airway pressure with periodic short term reductions of the elevated airway pressure to a pressure magnitude no less than ambient atmospheric pressure.
U.S. Pat. Nos. 5,245,995 5,199,424, and 5,335,654, and published PCT Application No. WO 88/10108 describes a CPAP apparatus which includes a feedback/diagnostic system for controlling the output pressure of a variable pressure air source whereby output pressure from the air source is increased in response to detection of sound indicative of snoring. The apparatus disclosed in these documents further include means for reducing the CPAP level to a minimum level to maintain unobstructed breathing in the absence of breathing patterns indicative of obstructed breathing, e.g., snoring.
Bi-level positive airway therapy for treatment of sleep apnea and related disorders is taught in U.S. Pat. No. 5,148,802. In bi-level therapy, pressure is applied alternately at relatively higher and lower prescription pressure levels within the airway of the patient so that the pressure-induced patent force applied to the patients airway is alternately a larger and a smaller magnitude force. The higher and lower magnitude positive prescription pressure levels, which will be hereinafter referred to by the acronyms IPAP (inspiratory positive airway pressure) and EPAP (expiratory positive airway pressure), may be initiated by spontaneous patient respiration, apparatus preprogramming, or both, with the higher magnitude pressure (IPAP) being applied during inspiration and the lower magnitude pressure (EPAP) being applied during expiration. This method of treatment may descriptively be referred to as bi-level therapy. In bi-level therapy, it is EPAP which has the greater impact upon patient comfort. Hence, the treating physician must be cognizant of maintaining EPAP as low as is reasonably possible to maintain sufficient pharyngeal patency during expiration, while optimizing user tolerance and efficiency of the therapy.
Both inspiratory and expiratory air flow resistances in the airway are elevated during sleep preceding the onset of apnea, although the airway flow resistance may be less during expiration than during inspiration. Thus it follows that the bi-level therapy as characterized above should be sufficient to maintain pharyngeal patency during expiration even though the pressure applied during expiration is generally not as high as that needed to maintain pharyngeal patency during inspiration. In addition, some patients may have increased upper airway resistance primarily during inspiration with resulting adverse physiologic consequences. Thus, depending upon a particular patient's breathing requirements, elevated pressure may be applied only during inhalation thus eliminating the need for global (inhalation and exhalation) increases in airway pressure. The relatively lower pressure applied during expiration may in some cases approach or equal ambient pressure. The lower pressure applied in the airway during expiration enhances patient tolerance by alleviating some of the uncomfortable sensations normally associated with mono-level CPAP.
Although mono-level, bi-level and variable positive airway pressure therapy has been found to be very effective and generally well accepted, they suffer from some of the same limitations, although to a lesser degree, as do the surgery options; specifically, a significant proportion of sleep apnea patients do not tolerate positive airway pressure well. Thus, development of other viable non-invasive therapies and better versions of existing therapies has been a continuing objective in the art.
In this regard, even the more sophisticated CPAP apparatus heretofore known in the art, including those described in U.S. Pat. Nos. 5,245,995 5,199,424, and 5,335,654, and published PCT Application No. WO 88/10108, suffer from certain operational disadvantages which stem from the structural relationships of their essential components.
More particularly, the CPAP apparatus disclosed in U.S. Pat. Nos. 5,245,995 5,199,424, and 5,335,654, and published PCT Application No. WO 88/10108 provide feedback/diagnostic systems including at least one sensor (typically an audio transducer such as a microphone) in communication with the patient's respiratory system. This sensor is located on or is connected to means (such as a breathing mask or nasal prongs) in sealed air communication with a patient's respiratory system. The sensor continuously senses the patient's breathing patterns and transmits signals indicative of those patterns to information processing means which control the motor speed of a blower. The breathing pattern signals can also be continuously monitored and/or recorded, thereby imparting to the apparatus a diagnostic as well as therapeutic capability.
The blower delivers pressurized air to the patient through a length of conduit and the breathing mask or nasal prongs. When the sensor detects breathing patterns indicative of obstructed breathing, e.g., snoring, it transmit signals corresponding to this condition to the information processing means which causes an increase in blower motor speed and, therefore, blower pressure output, until unobstructed breathing is eliminated. The system also includes logic whereby blower motor speed (and blower pressure output) is gradually decreased if unobstructed breathing patterns are detected over a preselected period of time. The purpose of this feature is to provide the patient with a pressure minimally sufficient to maintain airway patency during unobstructed breathing, thereby enhancing patient comfort and therapy compliance.
Despite the general effectiveness of these apparatus, however, the structural relationship of their feedback/diagnostic system with respect to the patient's breathing circuit (i.e., the blower, gas delivery conduit, and breathing mask or nasal prongs) results in an arrangement of lesser reliability than would otherwise be desirable.
For example, certain feedback/diagnostic systems utilize a breathing pattern sensor mounted on or connected to the breathing mask or nasal prongs. Such an arrangement requires a length of feedback conduit to be added to the patient's breathing circuit. The feedback conduit extends from the breathing patterns sensor at the mask to the blower.
Such an added feedback conduit renders the patient's breathing circuit cumbersome and increases the risk of entanglement of the feedback circuit. The arrangement also increases the risk of the feedback conduit becoming kinked or having the conduit accidently disconnected from the breathing mask, either of which render the device inoperable. Such a feedback conduit also requires frequent cleaning because it is in contact with the patient's expired air.
An advantage exists, therefore,. for an apparatus for delivering pressurized air to the airway of a patient which includes a feedback/diagnostic system of higher reliability and increased ease of use, whereby diagnostic accuracy and patient comfort and adherence to the therapy administered by the apparatus are optimized.
A problem associated with positive airway pressure devices is a lack of moisture in the air delivered by these devices has a drying effect on patient airways which causes the patient to have considerable discomfort and difficulty sleeping.
Humidifiers have been developed for use with CPAP devices to humidify the air supplied to the patient. In the type of system according to the present invention in which the sensor is situated generally at an end of the breathing circuit remote from the patient any type of accessory such as a humidifier may attenuate or absorb snore sound.
Humidifiers for use with CPAP apparatus are taught in U.S. Pat. Nos. 4,807,616 and 5,231,979. Other humidifiers of interest are manufactured by Respironics, Inc. of Murrysville, Pa. and Healthdyne Technologies. However, these humidifiers are for use with conventional CPAP apparatus and therefore are not configured to acoustically tune snoring sound as required for use with the unique sleep apnea treatment apparatus of the present invention.
An advantage exists, therefore, for a humidifier which is configured to acoustically tune the snoring sound received from a patient in order to set the resonant frequency of the snore sound.